1. Field of the Invention
The present invention relates to a method for producing a high tensile strength and high toughness metal bend pipe.
In recent years, a pipe-line transportation system has been increasingly used as mass-transportation means for liquid and gaseous fuels in view of economy and safety, and along this tendencies demands have been increasing for higher tensile strength and higher toughness of materials used in the pipe-line transportation system.
Particularly, bend pipes used in bent portions of the pipe-line are subjected to severer service conditions than the service conditions to which straight pipes are subjected, and stress imposed to the bend pipes is more complicated.
Description of Prior Art
As for a method for producing a bend pipe there has been conventionally known a mandrel method and a high frequency method. The mandrel method has been confronted with by problems such as shape defects, irregular pipe quality, and increased production cost, and the high frequency method has defects such as non-uniform mechanical properties between the bent portion and the non-bent portion. Thus, these conventional methods have been unsuccessful in providing a high tensile strength and high toughness bend pipe which can stand for severe service conditions at low temperatures.
Reasons for the failures of these conventional methods may be explained as below.
In the conventional mandrel method, after the steel pipe processing, the steel pipe is heated to austenitize the steel, subjected to bending by a mandrel, heated again to 900.degree.C, and then quenched in water and tempered. Thus this method is susceptible to non-uniform quality due to the irregular quenching effect in the bent portion, as well as to shape deficiency.
Meanwhile, in the conventional high frequency method, after the steel pipe processing, the steel pipe is heated to austenitize the steel, then subjected to bending, and cooled in air. Thus, this method fails to give satisfactory strength and toughness as comparable with X-52 steel grades.
Further in order to obtain a bend pipe equal to or better than X-60 steel grades, the steel after the pipe processing is heated for austenitization, subjected to bending and the bent portion is quenched in water and tempered. Thus the heat cycle to which the non-bent portion is subjected is only the temper treatment, while the bent portion has a quenched and tempered structure, so that the steel pipe as a whole has a non-uniform quality, which causes stretcher reduction (SR) embrittlement in the bent portion. Further, the portion between the bent portion and the non-bent portion becomes a binary heated phase, a part of which is embrittled by the tempering treatment.
Also the welded portion during the pipe processing and the bent portion have different composition and structure due to the quenching and the tempering, thus causing embrittlement.